1. Field of the Invention
This invention relates to endodontic instruments having non-circular cross sections, and more specifically, to endodontic instruments for removing pulp tissue and widening root canals, which include cutting surfaces and non-contact surfaces spaced circumferentially about the instruments for decreasing stress and permitting debris to be directed away from the cutting surfaces of the instruments.
2. Related Art
When a diseased root canal (diseased pulp tissue) is found in the root canal of a patient's tooth, a dentist removes the pulp tissue in order to get access to the most apical end of the tooth where bacteria is normally found. Usually a series of files or reamers (hand instruments or rotary instruments) which have a circular cross section are used to both remove the pulp tissue and to widen the root canal. After the tissue is removed, irrigation solutions like Sodium Hypochlorite (NaOCl) are used to kill any remaining bacteria. Then the root canal is filled with gutta-percha and adhesive and sealed-off with such root canal preparations as the one sold by Essential Dental Systems, Inc. under the trademark EZ-Fill. Lastly, a crown could be fitted to the tooth.
The traditional endodontic instrument used to remove pulp tissue or dentin, in the root canal is circular in cross section. As the instrument engages the dentin, it creates a contact surface extending 360 degrees about the narrowest part of the canal. This complete circumferential contact creates resistance to the rotation of the instrument as it removes dentin. The resistance is transferred to both the instrument and the tooth in the form of stress. The greater the resistance, the greater the stress. Because the tooth is much thicker than the reamer or file, it has a minimum chance of distorting or fracturing. The reamer or file, which can be hand or rotary driven however, can easily distort by unwinding when engagement along the shank produces excessive torque generated by either hand or motor powered rotation. In fact, if rotation with excessive torque continues, the stainless steal reamer can unwind and then fracture. Ni--Ti (Nickel--Titanium) alloy instruments will generally fracture much more abruptly given the same amount torque because the Ni--Ti alloy is more flexible but failure can occur quickly and unpredictably.
One way to reduce the chances of excessive engagement is by using a sequence of ever thickening instruments that remove the dentin in a gradual fashion. In theory, each preceding reamer or file widens the canal enough to allow the incremental removal of dentin with the subsequent reamer or file never creating sufficient engagement for distortion during rotation. This technique is known as step-back because the wider reamers or files are also taken to a shallower depth to further minimize stress and distortion to the instrument.
Another technique employed in endodontics is called crown-down. It employs wider instruments first to open up the coronal aspects of the canal, and subsequentially thinner reamers and files are placed more and more apically. Both techniques attempt to reduce the amount of torque generated by limiting the degree of dentin engagement that the reamers and files encounter, to prevent excessive engagement of dentin which leads to distortion and potential fracture of the instruments.
However, the efforts by others in the past have not resulted in an endodontic instrument which reduces in stress in operation based on the geometry of the cutting surfaces of the instrument in relation to non-contact areas positioned circumferentially about the instrument.
Accordingly, what is desirable, and has not heretofore been developed, is the provision of an endodontic instrument which does not make full engagement with the dentin in a root canal during use so that stress is minimized and debris can be directed away from the cutting surfaces of the instrument.
Past efforts in this area, which have not been completely successful in providing stress-reducing endodontic instruments, include the following:
Heath, et al., U.S. Pat. No. 5,464,362, discloses a method for fabricating an endodontic instrument by a machining operation wherein a wire-like rod, composed of titanium alloy, is created with a high degree of flexibility, high resistance to torsional breakage, and sharp cutting edges along the working length. Said cutting edges having either a triangular or helical cross-section.
Weissman, U.S. Pat. No. 4,990,088, discloses a multi-functional dental tool having a first pointed end with a first shank cutting section and second, minor diameter shank section for reaming or drilling. Located immediately adjacent to the uppermost second section is a divergent counterbore section. Surrounding the maximum diameter of the counterbore section is an annular cleaner grinding surface and there above a top cutting section extending a relatively short distance above the grinding surface. The cutting portions of the tool include a lower drilling end, a counterbore portion, a larger routing section, and a top cutting section. The tool can be formed having a cross section of various regular polygons as desired, the apices of the polygons providing the cutting edges. The invention further discloses a combination dental tool wherein the shape of a lateral cross section of the first shank is a square or an equilateral polygon.
Arpaio, Jr. et al, U.S. Pat. No. 4,934,934, discloses a dental instrument adapted to be used as a dental file and reamer for removing dead or damaged tissue from the root canal of a tooth. The instrument is formed, having at least two helical flutes defining at least two continuous helical cutting edges. A helical peripheral end extends between the helical flutes at the periphery of the shank. The invention discloses an instrument having three or four continuous helical flutes and having helical cutting edges at the periphery of said working portion of said shank equally spaced about the circumference of said shank. The cross-sectional shape of one embodiment is triangular.
Martin, U.S. Pat. No. 4,661,061, discloses a dental instrument for performing root canal work. The end or working portion of the instrument is connected to the instrument shaft and is provided with four faces of like size, shape, and orientation, forming a generally square shaped cross-section. The four faces are joined to one another by four edges which meet at a point and are curved in an unbroken line toward each other, whereby when the canal is being rasped the debris is forced out of the canal away from the instrument end.
Leonard, U.S. Pat. No. 4,634,378, discloses a dental drill having a triple helicoidal flute and progressive pitch so that the free spaces between the cutting lips increase progressively as they move away from the top of the drill towards the shank. The flutes have cutting lips along peripheral edges thereof for drilling dental canals, and triple flutes at each cross-section of the drill.
Groves, et al., U.S. Pat. No. 4,260,379, discloses an endodontic instrument, whereby the file or reamer portion is manufactured by twisting an elongated tapered parallelogram ground bar. The resulting twisted configuration provides a file with a major diameter approximately corresponding to the long diagonal of the parallelogram and a minor diameter corresponding to the short diagonal of the parallelogram. The spiral edges defining the large diameter sections provide the cutting edges whereas the second set of alternating edges defining the reduced diameter sections provide a retaining and debris removal function during operation of the instrument. The result is an endodontic instrument with increased strength and stiffness compared to traditional endodontic instruments.
Daniel, U.S. Pat. No. 1,527,845, discloses an improved toothpick with a finger grip element of compressible material anchored to the stock by an attached wired element. The toothpick has a flattened end which can be used as a scraper.
Fowler, U.S. Pat. No. 1,067,015, discloses a dental broach which is constructed of flexible steel in order to serve as a nerve extractor and reamer, whereby such construction minimizes the risk of the broach binding or breaking in a root canal. The broach consists essentially of a round, spring-tempered wire. The broach is flattened at one end to form a generally rectangular cross-section, and placed in a suitable holder and by any suitable means turned or rotated to twist the flattened portion while forming a continuous double coil or spiral having continuous longitudinal reaming or cutting edges. This produces an extracting and reaming surface which has a screw-like action when the broach is rotated in a clockwise direction, thus adapting the broach for use in penetrating and cleaning out very small canals without the pressure usually required from conventional broaches.
None of the previous efforts, taken either alone or in combination, teach or suggest the provision of endodontic instrument having a non-circular cross section which includes one or more cutting surfaces and one or more non-contact areas positioned circumferentially about the instrument for reducing stress during the removal of dentin from a root canal, and for directing debris away from the cutting surfaces of the instrument.